Electrolytic condenser having single crystal anode approaching purity



W. K. HOOPER Aug. 23, 1966 3,268,777 ELECTROLYTIC CONDENSER HAVING SINGLE CRYSTAL ANODE APPROAGHING PURITY Filed March 9, 1962 m -K. Hoop r INVENTOR.

BY fi/oflz g Hafiz Menus 3,268,777 Ice Patented August 23, 1966 3,268,777 ELECTROLYTIC CONDENSER HAVING SINGLE CRYSTAL ANODE APPROACHING PURITY William K. Hooper, Brookfield, Conn, assignor to Republic Foil, Inc., Danbury, Conn., a corporation of Delaware Filed Mar. 9, 1962, Ser. No. 178,618 1 Claim. (Cl. 317-230) This invention relates to electrolytic condensers, particularly to an improved aluminum electrode in same.

Structurally, electrolytic condensers consist of a metallic annode, an oxide layer dielectric on the metallic anode and a Wet or dry electrolyte acting as a cathode. The anode metal generally used is tantalum or aluminum. Since the trend in the electrical component industry is towards higher eificiencies, various improvements have been made in the prior art to increase the capacitance value of electrolytic condensers while reducing their bulk.

The capacitance of a condenser is directly proportional to the area of its electrodes, in this case the area of the anode, and for this reason it has been a long time practice in the industry to use foil having the possibly largest area in the possibly smallest space. It has been found that the effective area of the anode can be increased by a ratio of approximately 6:1 to 10:1 by processes such as electrolytic or chemical etching, whereby the formerly smooth surface of the anode foil loses its smoothness and becomes mat, resulting in an effective increase of the anode surface area.

The capacitance of a condenser is inversely proportional to the thickness of the dielectric. It is for this reason that the thickness of the oxide layer, generally formed by anodic oxidation of the electrode surface, is preferably kept thin; e.g. in the case of aluminum approximately 0.02 to 0.04 microns. The formation of a good quality oxide layer is highly important, especially in the case of such thin layers, to obtain a homogeneous, adhering, uniform and non-porous oxide coating.

There is a serious limitation as to the magnitude of the voltage used in the formation of the oxide layer, in order to avoid this layer becoming excessively thick. Due to the practical limits of the forming voltage and the increased difliculties in obtaining a layer of desired quality because of the non-smooth surface of the metal, the industry has been constantly hampered by the quality of the oxide layers which fell short of the desired characteristics. The quality of the oxide layer has a direct bearing on the life characteristics of the condenser, manifesting itself in changes in the power factor and leakage current. A great degree of caution has to be used in the selection and purity of the electrolyte to prevent even the smallest trace amounts of corrosive contaminants from being encapsulated in the capacitor.

Non-uniformities in the surface of the etched electrode have plagued the industry for a considerable time and, therefore, various methods have been devised to anneal or heat treat the anode foils to better distribute the inhomogeneities therein. These treatments have led to no substantial improvements and, therefore, electrolytic condensers with aluminum anodes do not enjoy as great popularity as ones having tantalum anodes, which metal can be treated with more predictable and reproducible results. The problem in the case of tantalum electrodes is that the price of this metal is fifty to seventy five times higher than the price of aluminum, resulting in obvious commercial disadvantages.

It is an object of the invention to provide an improved aluminum anode for electrolytic condensers without the attendant drawbacks of the prior art.

The so-called high-purity aluminum heretofore used in the manufacture of aluminum electrodes is 99.99% pure. According to the invention, it was found that by using an aluminum anode having a purity in the order of magnitude of 99.9999% most of the above-mentioned problems of alumin um electrodes in electrolytic capacitors can be eliminated.

The high purity, 99.99% aluminum has discontinuities in the form of minute impurities at the grain boundaries. Since these contribute to the deforming of a capacitor, the ultra pure 99.9999% aluminum which can be made either in the form of single crystals or by eliminating the impurities to the desired degree by various methods of refining, a substantially improved capacitor, having a desirably low leakage current, longer shelf life, resistance to deforming, usable between :greater temperature limits and requiring less care to exclude corrosive contaminants from the electrolyte, can be produced.

The effective electrical resistivity of a metal can be derived from two parts: the theoretical resistivity obtained under an ideal thermal agitation of the metal atoms and the residual resistivity which is partly due to impurities and partly to crystalline imperfections. It can be readily appreciated therefore, that particularly at low temperatures, a considerable improvement in the resitivity properties can be obtained with ultra-high purity aluminum electrodes according to the invention. Experiments have also shown a vastly increased improvement of resistance to anodic corrosion of the proposed ultra-high purity aluminum over the known high-purity aluminum.

Reference is made to the sole figure of the drawing showing an enlarged, broken away section of an electrolytic capacitor, the latter being shown in phantom. An aluminum foil 1 is covered with a thin dielectric oxide layer 2. The dielectric layer 2 of the electrode 1 is embedded in an electrolyte 3, which acts as the cathode of the electrolytic capacitor.

The beneficial effects of the ultra pure aluminum anode can be observed when the aluminum concentration is 99.999% or greater. A purification of the met-a1 can be accomplished by a number of known methods. Most of these methods utilize the process of impurity concentration.

A well known method of impurity concentration is the so-called zone refining process, wherein a bar of metal is refined by passing a narrow liquid zone across a metal bar in such a way that the zone is formed at one end of the bar and proceeds along its length to the other end at a slow rate. During the solidification, the impurities are pushed ahead of the advancing solid-liquid interface and diffuse away into the liquid zone at a very slow and uniform rate. This is due to the fact that the impure metal has a lower melting point and that most impurity atoms remain in the liquid phase leaving a purer metal at the solid liquid interface. As a result, the end portion of therefined ingot will contain most of the impurities leaving behind a refined bulk of material. This process can be repeated several times whereby further impurities will be swept out of the bulk. The number of zone refining cycles will depend on the starting metal, the required purity and the variety of contaminant.

The ultra pure aluminum electrode can be used either as a single crystal whereby crystal boundaries and domains are eliminated or in an ultra pure polycrystalline form.

There are a great variety of electrolytic capacitors in which an electrode according to the invention can be bene ficially used. Furthermore, a number of techniques are available by which the desired degree of purity and 3 4 crystallinity of the metal can be accomplished, therefore, References Cited by the Examiner the scope of the invention is to beinterpreted from the appended claim UNITED STATES PATENTS What is claimed is: 1,997,562 4/1935 Rhodes 317230 An electrolytic condenser comprising an electrode, a 5 2 1 35 3/1939 pavelka dielectric layer on said electrode, and an electrolyte, the electrode consisting of a single crystal of from 99.999 JOHN W. HUCKERT, Primary Examiner. to 99.9999 percent by weight aluminum. J KALLAM, A i n Examiner. 

